Why the deep parsing of the page gives an error?

Party flagma.ru. It became interesting to analyze this website. All works well up to 5 pages inclusive, then comes this error:
http://-//W3C//DTD%20XHTML%201.0%20Strict//EN
Ie, make a request on page 6 and opens it. What does that mean? Clearly it's not in the code because the rest of the pages to 5 work.

also noticed in the code of every page there is something like a bug in doctype.

p.s. doing googlescript
July 12th 19 at 17:17

Find more questions by tags GoogleJavaScriptParsing